1. Field of the Invention:
This invention relates to color video cameras and, more particularly, to a colorimetric circuit for white balance adjustment wherein the colorimetric sensor portion is provided with a plurality of sensor elements.
2. Description of the Related Art:
Of the conventional color video cameras capable of automatically adjusting the white balance in accordance with the ambient color temperature of an object to be photographed, the widely known type is that wherein the dichromatic sensor responsive to the red and blue primaries is used for detecting the color temperature in terms of a ratio of the red and blue components (or R/B). The gain control for the chrominance signal is based on this information.
But, under such an illumination as with fluorescent lamps, because the green component is strong, the use of the red- and blue-responsive sensor elements alone leads to a difficulty of accurate adjustment of the white balance.
So, a trichromatic sensor responsive to the red, green and blue primaries is instead used to yield R/G and B/G. By adjusting the gain of the amplifier for the chrominance signal in accordance with these two ratios, it is possible to obtain video signals of correct color balance even when the illumination light source has a special distribution of energy throughout the spectrum such as that of the fluorescent lamp. A video camera employing this technique is disclosed, for example, in U.S. Pat. No. 4,616,253.
However, because such prior art camera makes use of a separate arrangement of a plurality of colorimetric sensors, the influence of an error caused by the difference between base outputs of the sensors is not negligible.
Particularly, change of the temperature affects the difference between the response characteristics of the amplifiers for logarithmic compression of the outputs of the respective sensors. Hence, the color balance is liable to error. There is another drawback in that a differential amplifier of high precision accuracy has to be used.
One method of eliminating these drawbacks is disclosed in Japanese Laid-Open patent application No. Sho 60-94589.
Its constructional feature is that the outputs of the color sensors are selected for successive application to one logarithmic compression amplifier whose output is then, in timed relationship, stored in a memory of a microcomputer. By virtue of computation, the aforesaid necessity of using the differential amplifier is removed and the possibility of occurrence of a difference between any two of the amplifiers is reduced to zero.
In this case, however, because it is in a time-displaced relation that the outputs of all the colorimetric sensors are sampled, if the light source includes ripples, an error is produced.